Beams, trusses, joists, and columns are the typical structural members that support the weight or loads of structures, including buildings and bridges. Structural members may be manufactured from a variety of materials, including steel, concrete, and wood, according to the structure design, environment, and cost.
Wood structural members are now typically manufactured from multiple wood segments that are bonded together, such as in glue-laminated members, laminated veneer lumber, parallel strand lumber and I-beams. These manufactured wood structural members have replaced sawn lumber or timbers because the former have higher design limits resulting from better inspection and manufacturing controls. Wood is a desireable material for use in many structural members because of its various characteristics, including strength for a given weight, appearance, cyclic load response, and fire resistance.
In any application, a load subjects a structural member to both compressive and tensile stresses, which correspond to the respective compacting and elongating forces induced by the load in opposite sides of the member. By convention, a neutral plane or axis extends between the portions of the member under compression and tension. The structural member must be capable of bearing the compressive and tensile stresses without excessive strain and particularly without ultimately failing.
Wood structural members have generally similar stress characteristics in tension and compression. A characteristic of wood structural members under extreme loads, however, is that ultimate failure in bending is usually initiated by failure in the tension portion due to localized defects such as knots, slope of grain, or finger joints. By comparison, the compression portion can withstand higher applied loads because columnar action or plastic shift in that portion of the member allows larger loads without initiating ultimate failure of the member. Accordingly, the conventional practice is to manufacture wood structural members to have adequate tension portions to bear the required tensile stresses in bending, which include a predetermined margin of safety.